96 THE SOCIAL LIFE OF ANIMALS 



fishes can strain them out of the water. When such 

 particles are removed by filtering, the growth-promot- 

 ing power of the conditioned water is greatly les- 

 sened, but it is not completely lost. In our experi- 

 ments we found that suspended food particles ac- 

 counted for 80 per cent or more of the increased 

 growth in conditioned water over that given in clean 

 control water. 



These experiments give certain suggestions con- 

 cerning some other conditioning factors that may be 

 acting. For example, we know that the skin glands of 

 fish secrete slime (Figure 15). When we have made a 

 chemical extract of this material we have frequently 

 recovered a growth-promoting substance, apparently 

 a protein, which was effective in stimulating growth 

 when diluted 1 to 400,000, or even 1 to 800,000 times. 

 At these dilutions it is not probable that this factor 

 is affecting growth by furnishing food material. 



There are, of course, other possibilities, many of 

 which we have checked. The increase in growth is 

 not due, for example, to a change in the total salt 

 content of the water, for this does not change in our 

 experiments; nor to differences in acidity or oxygen, 

 nor, so far as careful quantitative analyses have re- 

 vealed, to changes in chemical elements present. We 

 may be dealing with some sort of mass protection, 

 such as was discussed in the last chapter, in which 



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